Modulation transfer function measurement of microbolometer focal plane array by Lloyd's mirror method

Abstract

Today, both military and civilian applications require miniaturized and cheap optical systems. One way to achieve this trend consists in decreasing the pixel pitch of focal plane arrays (FPA). In order to evaluate the performance of the overall optical systems, it is necessary to measure the modulation transfer function (MTF) of these pixels. However, small pixels lead to higher cut-off frequencies and therefore, original MTF measurements that are able to extract frequencies up to these high cut-off frequencies, are needed. In this paper, we will present a way to extract 1D MTF at high frequencies by projecting fringes on the FPA. The device uses a Lloyd mirror placed near and perpendicular to the focal plane array. Consequently, an interference pattern of fringes can be projected on the detector. By varying the angle of incidence of the light beam, we can tune the period of the interference fringes and, thus, explore a wide range of spatial frequencies, and mainly around the cut-off frequency of the pixel which is one of the most interesting area. Illustration of this method will be applied to a 640×480 microbolometer focal plane array with a pixel pitch of 17µm in the LWIR spectral region.

MTF measurements of a type-II superlattice infrared focal plane array sealed in a cryocooler

“…The resulting MTF measurement is sparse, as we tailored which frequencies are generated by the CSIG so we know which frequencies to look for in the image acquired by the IRFPA. This is a global method which assumes identical pixels across the FPA like other existing MTF measurement methods relying on interference, random pattern and speckle [13][14][15][16]. However, this hypothesis is acceptable because T2SL are very promising in term of uniformity.…”

“…As it represents the worst case scenario in term of blur within an image, we will use Eq. (14) to evaluate the worst impact on the MTF this 31Hz vibration can have. While the amplitude of those vibrations on the FPA cannot be retrieved with the sole accelerometer measurement, it is still possible to have a good estimation of the upper boundary of the blur.…”